The Enhancement of Steam Condensation Heat Transfer on a Horizontal Tube by Addition of Ammonia
The condensation heat transfer characteristics of ammonia-water mixtures on a horizontal smooth tube were investigated under a wide range of surface subcooling. For each ammonia vapor concentration (AVC), the experiment was performed at different vapor velocities. The research presented here concerns the external condensation of down-flowing ammonia-water vapors on a 100mm long cooled horizontal copper tube of an external diameter of 16mm. The results showed that, the condensation heat transfer enhancement had been achieved by addition of ammonia to water in some of the experimental surface subcoolings which was due to the Marangoni effect. With the change of the surface subcooling, the condensation heat transfer coefficient (HTC) revealed nonlinear characteristics with peak values. The maximum ratio of HTC compared to pure steam is 1.9 at an ammonia vapor concentration of 0.38%. The condensation HTC decreased notably with an increase in AVC. In addition, the HTC for all vapor mixtures increases with a rise in vapor velocity. Compared with previous studies concerning the condensation of ethanol-water mixtures, the enhancement of condensation heat transfer by the addition of ethanol was greater than ammonia. To investigate the enhancement of steam condensation heat transfer using additives, the condensate film instability and the percentage of diffusion thermal resistance in the ammonia-water and ethanol-water binary system were studied respectively. The results indicated that the condensate film in the ethanol-water binary system was much more unstable than that in the ammonia-water binary system. Using additives of the same vapor concentration, it was discovered that the percentage of diffusion thermal resistance of ammonia-water binary mixture was much higher than that of ethanol-water binary mixture. Furthermore, it was possible for the unstable condensate film to significantly decrease the diffusion thermal resistance enhancing the condensation heat transfer significantly.